The operation of a vehicle has been analyzed for development, specification of failure cause, and the like by making the vehicle actually run on a road, and simulating the situation of the road run with a test bench such as a chassis dynamometer.
For this purpose, it is preferable to, during a road run along a predetermined test running route, measure various states of a vehicle including the circumstances of the vehicle, such as outside temperature, altitude, and gradient as well as the states of the vehicle itself, such as an engine rotation number, throttle opening level, vehicle speed, brake pedal level, and emissions. For this reason, various units for measuring the emissions and the like, such as an exhaust gas analyzing unit and an altitude detecting unit, are mounted in the vehicle.
Meanwhile, among the circumstances, in particular, the gradient greatly affects a load (or driving force) acting on a vehicle, and therefore by accurately measuring the gradient during a road run, a load (or driving force) equivalent to the load can be put on a vehicle during a test on a test bench to thereby provide the simulation test on the test bench as an effective test closer to the road run.
As disclosed in JP-A2001-108580, as the altitude detecting unit, there has been known a unit adapted to, from pressures indicated by a pressure sensor, calculate altitudes at the time when a vehicle ran on a road, and gradients based on the altitudes.
This altitude/gradient calculation method using the pressure sensor has the advantage of being able to continuously acquire pieces of data without any break, but since pressure varies depending on weather, also has the disadvantage that values of the altitudes and gradients obtained from the pressures are lack in reliability.
On the other hand, in recent years, a satellite measuring system such as a GPS has been developed, and by mounting a receiver for the system (hereinafter referred to as a GPS receiver) in a vehicle, both of a position and altitude can be measured with considerable accuracy.
However, measurement data from the GPS receiver has the disadvantage that while a vehicle is running in a place where radio waves from a sufficient number of (e.g., four or more) satellites cannot be received, such as the shadow of a building or a tunnel, the reliability of a value of the measurement data is lost.
On the other hand, in the case of simply replacing measurement data from the GPS receiver with altitude data obtained from a pressure sensor in the section where the measurement data is unreliable, i.e., in the section where the GPS cannot perform measurement, stepwise difference appears in measured altitude at the point of switching between the altitude data by the GPS and the altitude data by the pressure sensor. As a result, a value of a gradient calculated from a distance differential of the measured altitude exhibits an extremely high (or low) value far from a true value at the time of the switching.
The occurrence of the large error in the value of the gradient as described is not preferable because the gradient is one of the most important parameters for reproducing a road run on a test bench.